Pop music played a crucial role in the national debate over the Vietnam War. By the late 1960's, radio stations across the country were crackling with blatantly political songs that became mainstream hits. After the National Guard killed four antiwar demonstrators at Kent State University in Ohio in the spring of 1970, Crosby, Stills, Nash and Young recorded a song, simply titled "Ohio," about the horror of the event, criticizing President Richard Nixon by name. The song was rushed onto the air while sentiment was still high, and became both an antiwar anthem and a huge moneymaker.

A comparable song about George W. Bush's rush to war in Iraq would have no chance at all today. There are plenty of angry people, many with prime music-buying demographics. But independent radio stations that once would have played edgy, political music have been gobbled up by corporations that control hundreds of stations and have no wish to rock the boat. Corporate ownership has changed what gets played — and who plays it. With a few exceptions, the disc jockeys who once existed to discover provocative new music have long since been put out to pasture. The new generation operates from play lists dictated by Corporate Central — lists that some D.J.'s describe as "wallpaper music."

Recording artists were seen as hysterics when they complained during the 1990's that radio was killing popular music by playing too little of it. But musicians have turned out to be the canaries in the coal mine — the first group to be affected by a 1996 federal law that allowed corporations to gobble up hundreds of stations, limiting expression over airwaves that are merely licensed to broadcasters but owned by the American public.

When a media giant swallows a station, it typically fires the staff and pipes in music along with something that resembles news via satellite. To make the local public think that things have remained the same, the voice track system sometimes includes references to local matters sprinkled into the broadcast.

What my rock 'n' roll colleague William Safire describes as the "ruination of independent radio" started with corporatizing in the 1980's but took off dramatically when the Telecommunications Act of 1996 increased the number of stations that one entity could own in a single market and permitted companies to buy up as many stations nationally as their deep pockets would allow.

The new rules were billed as an effort to increase radio diversity, but they appear to have had the opposite effect. Under the old rules, the top two owners had 115 stations between them. Today, the top two own more than 1,400 stations. In many major markets, a few corporations control 80 percent of the listenership or more.

Liberal Democrats are horrified by the legion of conservative talk show hosts who dominate the airwaves. But the problem stretches across party lines. National Journal reported last month that Representative Mark Foley, Republican of Florida, was finding it difficult to reach his constituents over the air since national radio companies moved into his district, reducing the number of local stations from five to one. Senator Byron Dorgan, Democrat of North Dakota, had a potential disaster in his district when a freight train carrying anhydrous ammonia derailed, releasing a deadly cloud over the city of Minot. When the emergency alert system failed, the police called the town radio stations, six of which are owned by the corporate giant Clear Channel. According to news accounts, no one answered the phone at the stations for more than an hour and a half. Three hundred people were hospitalized, some partially blinded by the ammonia. Pets and livestock were killed.

The perils of consolidation can be seen clearly in the music world. Different stations play formats labeled "adult contemporary," "active rock," "contemporary hit radio" and so on. But studies show that the formats are often different in name only — and that as many as 50 percent of the songs played in one format can be found in other formats as well. The point of these sterile play lists is to continually repeat songs that challenge nothing and no one, blending in large blocks of commercials.

Senator Russell Feingold of Wisconsin has introduced a bill that would require close scrutiny of mergers that could potentially put the majority of the country's radio stations in a single corporation's hands. Lawmakers who missed last month's Senate hearings on this issue should get hold of the testimony offered by the singer and songwriter Don Henley, best known as a member of the Eagles, the rock band.

Mr. Henley's Senate testimony recalled the Congressional payola hearings of 1959-60, which showed the public how disc jockeys were accepting bribes to spin records on the air. Now, Mr. Henley said, record companies must pay large sums to "independent promoters," who intercede with radio conglomerates to get songs on the air. Those fees, Mr. Henley said in a recent telephone interview, sometimes reach $400,000.

Which brings us back to the hypothetical pop song attacking George Bush. The odds against such a song reaching the air are steep from the outset, given a conservative corporate structure that controls thousands of stations. Record executives who know the lay of land take the path of least resistance when deciding where to spend their promotional money. This flight to sameness and superficiality is narrowing the range of what Americans hear on the radio — and killing popular music.